20'x48" riversled, what to use for core?

You'll pay a dear weight penalty with that type of bottom. These plastics have limited uses and no real structural value, so it's just a heavy skid plate. The Xynole sheathing will serve to water proof the boat, acts as an abrasion layer and doesn't have the adhesion issues a UHMW or HDPE sheet would, plus would be a lot cheaper too.

 

How will the Xynole last sliding over rocks?

 

Xynole gets a lot of it's abilities from the substantial resin content, which is easily 3 times that of regular 'glass cloth. As a result it tolerates abrasion and impact well, though if high speed impact is expected, you should consider additional fabrics in the laminate.

Laminates like this are mostly based around the speed the hull will make, which is one of the determining factors to the laminate schedule. If it's fairly slow craft, Xynole will not need much if any additional reinforcement. On the other hand, if you're blasting along a 30 knots, you'll want a substantial laminate, because impact loads rise exponentially.

How heavy is it, how fast is WOT and what will the average speed be? Judging by the above image, it's not very heavy and I'm assuming you'll be poling around rather then screaming along. If this is the case, then a single layer of Xynole will do. 2 layers if you plan to really beat the crap out of it. If a high speed powerboat, you need to add reinforcements (Spectra, Kevlar, etc.).

 

WOT will be 15mph or so, weight is still up in the air. I want it to be as light as possible but still sturdy enough to get us down the river. Average speed will be whatever the current is, usually 2-3mph

 

If you hit a rock or hard bottom at 15 MPH, you'll do some damage, so 2 layers of Xynole would be a good bet, plus the sacrificial runners. Drifting along, a single layer will be more then enough.

 

99% of the time any rocks are hit by the lower unit on the motor.

 

Then a single layer of Xynole and a stout transom is what you need. Maybe an enlarged skeg on the lower leg.

 

what kind of resin do you recommend with the xynole?

 

Epoxy is the only resin suitable for this type of laminate and expected usage environment.

 

Yes it is.

"Colloida silica combinations with organic binders have mainly been used in inorganicorganic hybrid coatings in OEM and product finishes industries to improve their scratch and abrasion resistance and their durability to weathering."

http://ppgamercoatus.ppgpmc.com/techcenter/docs/Inorganic_organichybridcoatings.pdf

I have just been creating some surfaces with it, and even within the first 24 hours of curing, it takes some very rough abrasive to knock the high spots off.

The strength layers don't have the additives. You can put the silica additives on the top layer to create a hard surface.

I would be interested to see any technical papers on "bubbles" significantly reducing the benefits of silica - it is the first I have heard of it.

The standard comments about Silica and strength normally go like this :

"Additive : colloidal silica (for epoxy resins)
406

406 Colloidal Silica is a thickening additive used to control the viscosity of the epoxy and prevent epoxy runoff in vertical and overhead joints. 406 is a very strong filler that creates a smooth mixture, ideal for general bonding and filleting. It is also our most versatile filler. Often used in combination with other fillers, it can be used to improve the improve strength, abrasion resistance, and consistency of fairing compounds, resulting in a tougher, smoother surface. "

http://www.nauticexpo.com/prod/west...epoxy-resin-colloidal-silica-21588-48112.html

Aerosil 200 or Cabosil (thixotropic modifier) - Unthickened epoxy resins are not suitable for use adhesives, as bond line control is difficult. Thixotropic modifiers are added to thicken the consistency of the epoxy, with little effect on its physical properties

http://www.appliedpoleramic.com/specs/compositeboats.php

 

Polyethylene ? That requires mechanical fastening, so opens the possibility for water ingress doesn't it ?

 

Tests preformed by West, show decreases is strength in all hand and machine combined filler mixtures. In some areas of comparison you get gains, such as compressive strength, but with a comparative lose in flexural modulus (as and example).

Materials added on top of the wet resin aren't affected by this, understandably and wasn't part of the discussion, nor general use of filler materials. I use this technique in engine compartment enclosure structures frequently, applying aluminum oxide over freshly applied flood coats to improve fire proofing qualities as well as other applications, such as the inside of centerboard cases, where abrasion isn't the only concern.

Simply put and from a technical stand point, you'll lose and possibly gain (in some areas) with the addition of fillers in the resin.

West 403 (milled fibers) has about a 10% lose in compressive and flexural strength a gain in tensile modulus compared to straight resin of similar volume.

West 404 (calcium) about 7% lose in tensile and flexural strength, but gains in modulus (it's stiffer).

West 405 (wood flour/silica/cotton flock) about 4%

West 406 (silica) about 3%, with gains in hardness but an increase in brittleness.

West 407 about 25% in general loses.

Micro balloons about 30% in general loses.

Spheres about 37% in general loses.

The point of all this is, filler mixtures should be thought out and reinforcement materials selected, for the tasks anticipated. I think experienced users already know this and use a few types of structural glue and filleting mixtures as well as lighter, less strong fairing and smoothing mixes.

All too often I see folks using wood flour in fairing mixes or balloons to bulk up structural adhesives of fillets. Some of the mixtures I've seen are literally half as strong as they could be, just with the application of the wrong reinforcement materials. Professionals generally don't have these issues, mostly because they've adhered to a schedule or common set of mixtures, established by someone with a clue about physical properties.

A classic example of this is;
"406 Colloidal Silica is a thickening additive used to control the viscosity of the epoxy and prevent epoxy runoff in vertical and overhead joints. 406 is a very strong filler that creates a smooth mixture, ideal for general bonding and filleting. It is also our most versatile filler. Often used in combination with other fillers, it can be used to improve the improve strength, abrasion resistance, and consistency of fairing compounds, resulting in a tougher, smoother surface. "

The key comment here is:
"Often used in combination with other fillers"

Silica alone in the mixture will make a smooth and hard fillet, but it will be brittle, so you need something else to help in this regard, such as cotton flock or other "fibrous" material.

West is correct in that, I have some silica in most mixtures I make. It's a thixotropic agent and one of the best viscosity controls available. Percentage of course depends on need. The same might be true of milled fibers on 'glass repairs or alteration tasks, where I've found no other material as well suited, for tie in or bond to polyester hulls.

 

UHMW would certainly add a lot of toughness, but also weight (not to mention cost). I'd think that sacraficial runners + epoxy-saturated kevlar felt (like they use on canoe stems) in key locations would provide plenty of abrasion resistance.

If it were me, I'd rather do an inexpensive build that's easy and let it get torn up a little than try to build it indestructible. If you have to spend a few weekends building a new one a decade down the road, who cares?

You could pick something a little extra tough for your runners (like maybe white oak? or some tough plastic perhaps) and I have to imagine it would last a super long time. Seems like you'd have to run into a hell of a lot of stuff before you wore through a 3/4" white oak runner.

 

The best price I found on corecell was Easternburlap.com, about $155 per 4 x 8 sheet Plain M80 16mm.